Asthma has significant human health and economic impacts. Excessive mucus is an important cause of airflow obstruction in fatal asthma. It is also present in mild to moderate disease, but is poorly understood and treated. Mucus overproduction in asthma is associated with the dysregulated expression of two mucin glycoproteins - MUC5AC and MUC5B. Increased MUC5AC is a consistent finding, but MUC5B varies. It remains stably produced in some patients, but is strongly repressed in others (>90%), resulting in as much as 400-times more MUC5AC than MUC5B. This dichotomous expression pattern occurs in patients who display the strongest acute responses to the inhaled bronchoconstricting agent methacholine (MCh). This airway hyperreactivity (AHR) feature can be modeled in mice through allergen exposure. Knockout mice lacking Muc5ac are protected from AHR, indicating that it is a critical mediator of asthma-like airflow obstruction. Thus, determining specific mechanisms of MUC5AC-mediated AHR may reveal important mechanisms of obstruction and potential targets for improving airflow in asthma. As part of a long term goal of elucidating the functions of MUC5AC and MUC5B in the airways, Muc5ac and Muc5b knockout and overexpressing mice will be used to model dichotomous mucin expression in human asthma, and to test the specific effects of each mucin on the biophysical properties of mucus that promote obstruction. In addition to their differential expression, emerging data show that MUC5AC and MUC5B have distinct disulfide polymer structures. They also have specific glycosylation patterns: MUC5B is sialylated, MUC5AC is fucosylated. Fucosylation increases mucus viscoelasticity, and FUT2, the enzyme that catalyzes mucin ?1,2-fucosylation, is associated with asthma exacerbation risk. Thus, the concept driving this proposal is that mucins with distinct polymer and glycan structures establish the obstructive potential of airway mucus. Accordingly, the proposed studies test the hypothesis that MUC5AC mediates AHR through specific disulfide and fucosylation mechanisms that cause obstruction by promoting mucus viscoelasticity. Three Specific Aims are proposed: 1) demonstrate that MUC5AC, but not MUC5B, mediates acute AHR in allergic mouse airways; 2) determine whether increased mucus viscoelasticity is caused by MUC5AC-specific polymerization and fucosylation; 3) determine whether MUC5AC disulfide and fucosylation-dependent viscoelastic properties are critical mechanisms of AHR and mucus plugging. Wild type and mucin mutant mice will be studied at baseline, and under conditions of allergic inflammation induced by Apergillus oryzae extract exposure. Lung function, mucociliary clearance, and mucus viscoelasticity will be measured. The effects of pharmacological, enzymatic, and genetic inhibition of polymerization and fucosylation will be determined. Successful completion of these studies will advance our mechanistic understanding of AHR, and could identify novel strategies to prevent obstruction while preserving airway defense in asthma and other lung diseases.